1. Field of the Invention
The present invention relates to inertial measurement units. More specifically, the present invention relates to techniques for compensating for heat in inertial measurement units and other instruments.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
U.S. Pat. No. 4,711,125, issued Dec. 8, 1987, to Melvin M. Morrison and entitled "Inertial Measurement Unit" (the teachings of which are incorporated herein by reference) describes an inertial measurement unit which provides three axis acceleration and angular turning rate detection with a cubical magnetically suspended sensor mass disposed within a cubical outer assembly. The sensor mass is free to move mechanically independent from the outer assembly. The sensor mass has a plurality of sensing and suspension elements of particular orientation on a selected plane for each axis of detection which face a corresponding set of sensing and suspension elements respectively on interior surfaces of said outer assembly. The sensing elements are advantageously cross-coupled to minimize cross-axis sensitivity. The device utilizes several servo-control loops of conventional design to process outputs from the pickoffs and maintain the sensor mass in proper orientation.
The above-described inertial measurement unit (the "Cube"), with all of its advantages over existing conventional IMU's, has an inherent limitation with which could impact its accuracy and cost advantages. Since the Cube is a single instrument with six outputs, it is difficult to isolate the heating effects of the coils between channels.
The Cube contains 18 heat generating coils oriented symmetrically about the inside faces of the six outer plates with three coils on each plate. Each of the coils act as part of a force rebalance subsystem against corresponding fixed magnets on the sensor mass. When the sensor mass is displaced due to an accelerative force, an increase in direct current in the corresponding coil forces the sensor mass back to the null position. This increase in current causes an increase in the heat generated by that coil. This heat is transferred to the surrounding plate in which it is mounted through conduction, and to the sensor mass through radiation, convection and/or conduction.
The heat transfer causes a thermal gradient through the entire structure from the heat source (the coil) to the heat sink which is generally the outside structure of the Cube. The subsequent temperature increase causes the materials used to construct the outer plates and the sensor mass to expand. This expansion has the effect of changing the pick-off scale factor in that channel as heat slowly propagates throughout the structure. This effect is compounded by the fact that the pickoffs in all other channels are effected by the thermal gradients and subsequent expansion even though there are no accelerations in their respective channels. An output will be produced in every channel because of the heating effect and must be countered in some way. Furthermore, the thermal gradients are very dynamic during typical operation when the linear and angular accelerations shift from channel to channel making it very difficult to separate channel outputs due to heating from those caused by acceleration.
Conventional IMU's control intra-channel thermal effects by converting the DC output signal from analog to digital and compensating for the effects through software. This technique is very effective in conventional IMU's because the individual channels are isolated from each other. The thermal effects of one channel do not effect any other channel because the IMU consists of individual instruments which are physically separated from each other. This is not the case in the Cube since all channels are contained in the same instrument.
With 18 variable heat sources for the Cube, the thermal gradients are generally very dynamic and slow in response to the inputs. The number of combinations of overlapping thermal gradients across each of the 36 pickoffs are nearly unlimited. If the conventional approach is undertaken with respect to the Cube, i.e., compensation through software, the designer is faced with a tradeoff between the number of thermal gradient response scenarios and the response time of the computer, that is, the number of times per second the computer can respond to an acceleration input. Thus, modeling only a few basic scenarios will cause a loss in accuracy due to the pick-off scale factor changes which are not modeled. And a modeling of every scenario require a considerable amount of computer software and so many calculations that the response time of the Cube would be slowed considerably. Further, the development cost and time required to test the scenarios and write the software add to the overall cost of the system, not to mention the opportunity cost of a delayed entry into the market.
The system can be improved somewhat by adding heaters to the Cube so that it operates at a relatively high temperature. This causes the thermal gradients to be less severe. However, the heaters and the temperature sensor require an independent circuit, add cost to the system, do not eliminate the digital scale factor adjustments add to the number of adjustments to be compensated for.
What is needed is a thermally stable Cube design which would emit the same amount of heat regardless of the current drawn for forced rebalance of the sensor mass. If the heat generated by all 18 coils in the Cube is always the same, the thermal gradients in the structure will reach static conditions after a warm-up period. The structure supporting the pickoffs, as well as the pickoffs themselves, would expand to a static displacement position and thereafter operate at constant geometry. This would insure that there are no changes to the scale factor due to variable heating of the coils so an output in any channel will be purely from acceleration.
Furthermore, the exchange of radiant energy between the inside surface of the outer plates and the outside surface of the sensor mass would have a reciprocity relationship and would therefore be in thermal equilibrium. This would insure that the magnetic field strength of the fixed magnets do not vary due to temperature effects. Further, this would eliminate the need for complicated software to compensate for such scale factor changes and allow the Cube to run at maximum speed.
Thus, there is a need in the art for further improvements in the design of inertial measurement units. Specifically, there is a need for a thermally stable Cube design which would emit the same amount of heat regardless of the current needed for forced rebalance of the sensor mass.